Polysaccharides, in particular, galactomannans such as guar and hydroxypropyl guar, and xanthan and xanthan gum, have a variety of uses. Guar in the form of gum is used primarily in food and personal care products for its thickening property. The gum has five to eight times the thickening power of starch. Guar gum is also used as a fracturing aid in oil production.
Guar gum is the mucilage found in the seed of the leguminous plant Cyamopsis tetragonolobus. The seeds are composed of a pair of tough non-brittle endosperm sections, hereinafter referred to as guar splits. Guar splits contain guar gum but are tough and extremely difficult to grind into a powder form for recovery of the gum. After processing, native guar gum is obtained in the form of a yellow powder and has a molecular weight of between about 2,000,000 Daltons and 5,000,000 Daltons.
In certain applications forguargum, such as in food products and personal care compositions in oil well fracturing and petroleum recovery, it is preferred to use a relatively low molecular weight material for better performance. For example, in oil wells when used as a fracturing aid it is preferable that guar gum have a molecular weight of between 100,000 Daltons and 250,000 Daltons, as such lower molecular weight gum achieves better high fracture conductivity and low formation damage results in oil production operations. In addition, as guar gum used in oil field applications is modified by means of crosslinking additives as will be discussed below, the depolymerized guar must be capable of crosslinking.
Guar gums of lower molecular weights have been obtained by depolymerizing the native gum. One method of depolymerizing guar currently in use is through treatment with hydrogen peroxide. However, depolymerization via hydrogen peroxide treatment has the disadvantage that it is difficult to control so as to yield guar gum of a pre-selected range of molecular weights. More specifically, hydrogen peroxide treatment generally produces depolymerized guar gums having a polydispersity of between 3 and 5, which is too high. (Polydispersity is defined as the weight average molecular weight divided by the number average molecular weight of the treated guar.) The depolymerized guar gum used in oil well production should have a polydispersity value of no greater than about 3.0. This depolymerization method also causes the guar gum to form agglomerates with hydrogen peroxide, which reduces the purity of the depolymerized guar.
U.S. Pat. No. 5,273,767 relates to a method of preparing a modified rapidly hydrating xanthan and/or guar gum and sterilizing food products comprising xanthan and/or guar gum by irradiation. The irradiation may be carried out with a high energy electron beam at a level between about 0.1 and 4.5 Mrad.
U.S. Pat. No. 6,383,344 B1 discloses a method of reducing the molecular weight of polysaccharide polymers, in particular, hyaluronic acid and carboxymethylcellulose, by irradiation of the polymers. The specific forms of irradiation disclosed are by gamma rays and microwaves. The preferred form disclosed is gamma radiation. However, the use of gamma radiation requires rigorous safety precautions as gamma radiation generated from a radioactive source is highly toxic.
An article by King et al., entitled “The effect of Gamma Irradiation on Guar Gum, Locust Bean Gum (Gum Tragacanth) and Gum Karaya,” Food Hydrocolloids, Volume VI, No.6, pp.559-569, 1993, reports on treatments of galactomannans, such as guar gum, with low doses of gamma radiation. The resulting products were disclosed as having lower viscosities. The article indicates that viscosity of solutions of guar gum and locust bean gum decrease with increasing gamma irradiation dose when irradiated in dry powder form.
British Patent Publication No. 1,255,723 relates to the depolymerization of a water soluble cellulose ether by high energy electron beam irradiation. The process involves irradiating a layer of a free-flowing particulate, water-soluble cellulose ether, said layer having a uniform depth adjusted to within ten percent (10%) of the penetration depth of the beam. Cellulose ether is a substituted polysaccharide but not a galactomannan. This patent does not disclose the depolymerization of polymers to form a product having a preselected molecular weight range or a polydispersity value of less than about 3.0.
According to U.S. Pat. No. 5,916,929, irradiation of polymer materials yields two types of substantially different products. Some high polymers such as polyethylene and its copolymers, polybutadiene, polyvinylchloride, natural rubber, polyamides, polycarbonamides and polyesters, undergo molecular combination and eventually become crosslinked. Crosslinking essentially increases the molecular weight of a polymer and increases its melt viscosity, as measured by the melt flow rate, i.e., the numerical value of the melt flow rate decreases. A second class of polymers such as polypropylene, polyvinylidene chloride and fluorocarbon polymers, including polytetrafluoroethylene are known to undergo polymer degradation when irradiated with high energy ionizing radiation. This chain scissioning tends to decrease the molecular weight of the polymer, which is reflected by a decrease in the melt viscosity properties, as measured by an increase in the melt flow rate (MFR).